1. Technical Field
The present invention relates to silicalite and zeolite nanoparticles and, more particularly, to methods of preparing nonaqueous dispersions of such nanoparticles, to the dispersions so prepared, and to applications of the dispersions to produce interlayer dielectrics in the fabrication of integrated circuits.
2. Description of Related Art
Increasing the speed and performance of integrated circuits (xe2x80x9cICsxe2x80x9d) typically calls for increasing the density of electronic components on the surface of a semiconductor wafer and increasing the speed at which the IC performs its functions. Increasing component density brings charge-carrying circuit elements closer together, thereby increasing the capacitive coupling (crosstalk) between such circuit elements and delay in the propagation of signals through the conductors. Higher capacitance is detrimental to circuit performance, especially for higher frequency operation as would typically be encountered in telecommunication applications and elsewhere. One way of reducing capacitive coupling between proximate circuit elements is to reduce the dielectric constant (xe2x80x9ckxe2x80x9d) of the insulator or insulating material(s) separating the coupled circuit elements.
It has been conventional in the fabrication of ICs to use dense materials as dielectrics, including silicon dioxide, silicon nitride and cured silsesquioxanes among others. The dielectric constant of these materials typically lies in the range of approximately
kxcx9c3.0 to 7.0xe2x80x83xe2x80x83Eq. 1
The IC industry has entered a regime in which performance of ICs is typically limited by resistive-capacitive (xe2x80x9cRCxe2x80x9d) delay occurring in the metallic interconnects of the IC, indicating that lower k dielectrics will be required for future ICs. As yet, the only fully dense materials with k less than about 2.4 are fluorinated polymers or fully aliphatic hydrocarbon polymers. However, such materials have not been shown to have sufficient thermal and mechanical stability to survive the heat and mechanical stresses occurring during IC fabrication. In addition, these polymers typically have chemical properties that are similar in some respects to the chemical properties of photoresist materials commonly used in IC fabrication. Thus, selective chemical removal of photoresist layers and dielectric layers becomes more difficult.
Several candidate low k materials for IC dielectrics include materials having a high degree of porosity. The open structure of such porous materials includes a significant amount of airspace. Therefore, the overall effective dielectric constant of the material lies between those of air and the fully dense material, typically significantly lower than that of the pure, solid material. Several general classes of porous materials have been described, including porous silicon dioxides.
Previous work by one of the present inventors relates to the use of silicalite nanocrystals (xe2x80x9cSNCsxe2x80x9d) in forming spin-on dielectric coatings (interlayer dielectric, xe2x80x9cILDxe2x80x9d) in the fabrication of ICs as described in US patent application Ser. No. 09/514,966 incorporated herein by reference. Silicalites are porous crystalline forms of silica having the same crystal structure as zeolites, as described, for example, by Edith Flanigen and Robert Lyle Patton (U.S. Pat. No. 4,073,865). Colloidal suspensions of silicalite nanoparticles are described, for example, by Jan-Erik Otterstedt and Dale A. Brandreth, Small Particles Technology (Plenum Press, 1998), especially Chapt. 5. See also The Synthesis of Discrete Colloidal Particles of TPA-Silicalite-1 by A. E. Persson et. al. appearing in Zeolites, September/October 1994, pp. 557-567. SNCs offer the possibility of a porous, low k dielectric material that can easily be deposited on semiconductor wafers with standard wafer processing techniques and that can withstand subsequent etching, polishing and metallization steps.
However, SNCs are not suitable for ILD formation by themselves. A suitable binding agent must be used in cooperation with the SNC. That is, an SNC is typically deposited on the surface of a wafer along with a binding agent. Favored binding agents typically contain silicon and oxygen and crosslink at elevated temperatures, binding the SNCs into a porous ILD having adequate mechanical strength to withstand further IC processing. xe2x80x9cMonolithic filmsxe2x80x9d denote the films created by nanoparticles having been bound together by a binding agent. Binding agents based on silicon dioxide are desirable because of their proven compatibility with current IC processing steps, such as dielectric reactive ion etching and photoresist removal.
Silicate nanoparticles are typically made in aqueous solution and usually in a basic aqueous solution. However, typical silica-based binding agents used for forming ILDs from SNCs coagulate into a gel in the presence of water, especially so in the presence of water and base. Thus, fabricating practical ILDs from SNCs and binding agents must deal with the challenge of the catalytic effect of water in causing premature agglomeration of typical binding agents. The present invention relates to methods of redispersing SNCs from the basic aqueous solution in which they are typically made into a hydrophobic organic solvent while overcoming the tendency of SNCs to agglomerate and/or precipitate in nonaqueous solution. The process of fabricating ILDs from SNCs is thereby facilitated.
In addition to ILDs, other applications for monolithic films of silicalite or zeolite nanoparticles include filtration membranes, molecular sieve membranes and catalyzation membranes. See, for example, the review article by Anthony Cheethan, Gerald Ferey and Thierry Loiseau in Angewandte Chemie International Edition, Vol. 38, pp. 3268-3292 (1999).
The present invention relates to the formation of hydrophobic organosols of silicalite and zeolite nanocrystals (xe2x80x9cSNCsxe2x80x9d) which, among other applications, can be used in spin-coating of thin films for dielectric layers in integrated circuit (xe2x80x9cICxe2x80x9d) fabrication. SNCs are typically grown in an alkaline, aqueous medium, and deposited onto the IC, typically by spin-on deposition. However, SNCs deposited onto the IC wafer are insufficient to form a dielectric layer and need to be bound together into a porous solid, having sufficient mechanical and thermal stability to withstand further IC fabrication steps. Typically, a binder containing silica is used for coagulating SNCs into a porous, low k dielectric on the wafer surface. That is, a porous, siliceous binding agent is typically used along with SNCs to form silicalite dielectric layers. Typical binding agents include materials that polymerize or crosslink upon heating, thereby binding the silicalite nanocrystals together into a porous solid consisting substantially of silicon dioxide. However, typical binding agents are very reactive in the presence of water, and even more so in the presence of water and base. Thus, aqueous solutions of SNCs, especially alkaline aqueous solutions, quickly gel when typical binding agents are introduced. Thus, an objective of the present invention is to provide methods for removing water from the SNCs and to redisperse them in a hydrophobic solvent from which deposition along with binding agent is more readily accomplished than from aqueous solution.
Redispersal of SNCs into nonaqueous solvent(s) is preceded by chemical modification of the Sixe2x80x94OH bonds typically found on the surfaces of silicalite and zeolite particles prepared in aqueous media. In some embodiments, the chemical transformation of Sixe2x80x94OH on the surface of the SNCs is performed so as to form a direct silicon-carbon linkage (Sixe2x80x94(CH3)3, as one example). Other embodiments react Sixe2x80x94OH to form an ether linkage, Sixe2x80x94Oxe2x80x94R, referred to herein as xe2x80x9cetherification.xe2x80x9d Silicalites and/or zeolites thus modified can be dispersed in hydrophobic solvents that do not catalyze agglomeration when typical binding agents are introduced.